Forging metal powders

ABSTRACT

Alloy metal powders are forged over a controlled temperature range such that at the forging temperature the alloy powders are characterized by a microstructure comprised of at least two metallic phases which coact to retard grain growth during recrystallization.

United States Patent 1191 Church Jan. 22, 1974 [5 FORGING METAL POWDERS 3,702,791 11/1972 Freche et al 29/4205 x [75] Inventor: Nathan Lewis Church, Warwick, OTHER PUBLICATIONS l-Iirschom, J. S. et al., The Forging of Powder Metal- [73] Assignee: The International Nickel Company, y Preforms, In Metal Forming, P-

Inc New York NOV. I

[22] Filed: May 1972 Primary Examiner-,-Carl D. Quarforth [21] Appl. No.: 257,793 Assistant ExaminerR. E. Schafer Attorney, Agent, or Firm-Ewan C. MacQueen et aL [52] 11.8. CI 75/226, 29/4205, 75/200 1 51 1m. 01 B22p 3/14 [57] ABSTRACT [58] Field of Search 75/200, 226, 221; 29/4205; Alloy metal powders are forged over a controlled tem- 148/ 1 1,5, 12 perature range such that at the forging temperature the alloy powders are characterized by a microstruc- [56] References Cited ture comprised of at least two metallic phases which UNITED STATES PATENTS coact to retard grain growth during recrystallization. 2,809,891 10/1957 Ennor et al. 75/226 9 Claims, N0 Drawings 1 FORGING METAL POWDERS The subject invention is addressed in the main to powder metallurgy forging.

As the metallurgist is aware, powder metallurgy has for some time played a prominent role in the production of a number of useful structural components. This has been particularly evident in respect of products not readily responsive to the more conventional meltingcasting-working type processing,'notably the production of intricately shaped products, certain refractory metals and alloys, various dispersion hardened materials, etc.

The virtues of powder metallurgy notwithstanding, the customary compacting and sintering techniques nonetheless gave rise to an attendant porosity problem, i.e., a problem in which products as finally processed are characterized by voids which in turn significantly detract from various mechanical and/or physical properties, e.g., yield strength, the ability to absorb impact energy, etc. This difficulty has quite naturally served to restrict the scope and application of powder technology.

To be sure, a number of techniques have been devised to minimize this porosity drawback. Mention might be made of repressing and/or infiltration as well as conducting both the compacting and sintering operations at high temperature. Such procedures, however, do introduce added cost and/or do not lend themselves to mass production techniques.

In recent years powder metallurgy forging, an historically old and near forgotten art, has received considerable attention since it offers an economically attractive panacea for virtually eliminating porosity while being amenable to automation. To that end a primary objective of the present invention is to facilitate or otherwide advance powder forging in terms of processing technique and/or providing particularly useful forged prodnets.

in accordance herewith, it is contemplated that improvements in respect of powder forging can be brought about provided the forging operation is controlled such that it is conducted within a range of temperature at which the alloy to be forged is characterized by a special structure as herein described.

Generally speaking, the present invention involves the forging of powder alloys characterized at the temperature of forging by a microstructure in which the alloy powder particles contain at least two metallic phases, e.g., austenite and ferrite in the case of stainless steels or alpha and beta in the case of copper-zinc alloys, which mutually coact such that grain growth is retarded during recrystallization. Advantageously, at the forging temperature each of the phases is present in a volume percentage of at least about percent with their respective grains not exceeding a grain size of about ASTM 10 on average, the grain size most advantageously being not greater than about ASTM 12.

Upon bringing the alloys to the proper microstructural condition and then forging at a temperature which does not deleteriously disrupt this condition during forging, it is deemed that any one or more of a number of benefits follow, e.g., improved die filling, lower forging loads for a given configuration, less oxidation, the likelihood of decreased die wear, distortion, or breakage, reduced cost, etc. For example, with such structured alloys it is considered that lower flow stresses (less resistance to flow) obtain than might otherwise be the case. As a consequence, in various systems a significantly lower forging temperature can be used as opposed to the necessity of using one much higher. These and other such characteristics contribute to longer die life and less downtime as well. Even in the absence of a lower forging temperature, lower flow stress permits of reduced forging pressures and this can minimize die distortion.

With regard to the metallic phases, while the volume percentage of one or more such phases may be as low as 4 percent or 5 percent, (it being understood that one other such phases constitutes essentially the balance), in striving for an overall good combination of physical and/or mechanical properties, at least 10 percent, and most preferably about 15 percent or 20 percent or more, of at least two such phases should be present at the forging temperature. As a practical matter, the microstructures will, in most instances, be comprised of but two phases at the forging temperature. Thus, if one were present in an amount of but 1 percent or 2 percent, this would be unsuitable since, inter alia, there would be an insufficient percentage of the low volume phase to prevent undesirable grain growth in the other and, depending on composition, a higher flow strength than desired could obtain. In this connection and as indicated above, the grains of the phases at the temperature of forging should be at least about ASTM grain size 10 or finer. The grains can be larger, e.g., ASTM 8, but generally at the expense of some mechanical or physical property. A finer grain size, e.g., ASTM 14 or smaller, 'is considered particularly beneficial in respect of various properties. Grain size applies to the forged products as well.

Concerning the powder particles, while elemental powders may be blended and sintered to the desired composition, it is deemed preferable to use pre-alloyed powder. This can, for example, be accomplished through atomization in which a liquid melt of desired composition is formed and directly converted to powder by using air, steam, inert gas, vacuum or water to bring about atomization. Water atomization, with or without an inert gaseous stream, is considered appropriate since it is commonly employed and is relatively inexpensive. Prealloying and atomization also provide for small particle size and grain size. Moreover, it is preferred that the particles as alloyed be of irregular shape as opposed to, say, spherical. This enhances particle interlocking.

The alloy powders should not exceed about 500 microns, (including oxide film) preferably being less than 275 microns, an advantageous powder mix being one containing not more than about 25 percent of powder less than about 40 microns, the remainder being up to 225 or 275 microns. The surface of the particles is substantially, if not completely, comprised of an oxide film but this is beneficial in accordance herewith. This film inhibits grain growth within each particle as well as inhibiting growth across interparticle boundaries.

Though up to 1 percent oxygen can be present in the alloy powders, high oxygen can detract from certain properties as well as interfere with compacting and sintering prior to forging and also retards interparticle welding during forging. The oxygen level should not exceed about 0.15 percent or 0.25 percent and the oxide film should be less than about 5 or 10 microns thick.

chined, or aged in the case of The prealloyed powder particles are thereafter compacted to a preform, the shape of which will often, though not necessarily, depend on the shape of the final product. Thereupon, the prealloyed preform is heated to obtain the desired multiplex microstructure whereupon it is forged to shape, and full or nearly full density. As is oftendone in practice, an appropriate lubricant can be added to the prealloy powder before pressing to the preform. Also, the preform can be heated (sintered) prior to forging in accordance with usual practice. Subsequently, the product may, if desired and depending on composition, be further treated, e.g., ma-

age hardenable material etc.

It perhaps should be mentioned that preconditioning treatments are unnecessary as a prerequisite to obtaining a fine grain. In prior art wrought alloy processing it is common to significantly cold work a given alloy at room temperature and then heat into a plural phase region above the recrystallizationtemperature or work the alloy while cooling down to and into such region. This preconditioning is largely responsible for the fine grain; otherwise, a coarse grain results. Forging as contemplated herein involves virtually an isothermal operation during which continuous recrystallization takes place. This contributes together with other aspects such as atomization to maintaining a fine grain structure but without need of preconditioning. I

The following example illustrates carrying the invention into effect.

A melt of a nickel-base alloy nominally containing 37 percent chromium, 18 percent iron, 0.5 percent titanium, 0.05 percent carbon, 0.5 percent silicon, and the balance essentially nickel and impurities is prepared using conventional melt processing; The use of relatively pure materials is preferred together with vacuum melting. The melt is then atomized using gas, vacuum or high pressure water atomization to produce powder particles with sizes ranging from "-40 to 325 mesh. Each powder particle has substantially the composition of the parent alloy. High pressure water atomization is excellent for producing powder because it'is relatively inexpensive, produces an irregular particle shape which is readily compactable and is capable of producing powder with reasonably low oxygen content, e.g., less than about 0.15 percent oxygen by weight.

The resulting powders can be compacted into preforms (using a binder, if necessary, which is burned off by heating to a moderate temperature, e.g., 1,000 F), and then heated to a temperature as low as 1,800 F and up to about 2,200 F, although the temperature preferable is from 1,900-2,l00 F. If desired, the preforms may be held for some time at this temperature to provide some sintering of the particles or they may be placed into the forging die as soon as they reach the forging temperature. This forging step will provide densification of the preform and metal flow to fill the die cavity, to produce an accurate essentially completed product with nearly 100 percent density and mechanical and corrosion properties similar to those expected of the alloy it were in the wrought condition.

When the atomization process is complete, each powder particle will consist of a rapidly quenched gamma (face-centered-cubic, nickel-chromium solid solution with or without iron) phase. When the powder is reheated to about l,800 F. or above, fine particles of alpha phase (body-centered-cubic, chromium-rich,

e.g., 50 percent or more of chromium, solid solution containing nickel with or without iron) will be precipitated into the gamma phase. When in thisfine grained two phase condition, the alloy may be forged with various benefits. It is considered low flow stress will result at least in part from the dispersion of the lower strength (at 1,800 2.200 F.) alpha phase in the gamma matrix. This should enhance die filling, require less forging load, etc. The resulting forged product will have a desirable fine-grained, two-phase structure.

In considering various metallurgical systems and given the intended ultimate application of a forged powder product, a good balance can be achieved between ease of carrying out the forging stroke and mechanical properties. Upon heating to the multiplex region, in certain systems the first phase encountered is the weakest and thus offers low flow stress. Too, this phase would afford the opportunity to use the lowest forging temperatures; however, mechanical properties might also likely be the lowest. In such cases if the greater emphasis is on ease of forging then this low temperature weaker phase should predominate, i.e., be present above 50 percent by volume, say 55 to percent. On the other hand, where the greater emphasis would be on the mechanical properties coupled with ease in forging, then the reverse would hold true. Illustrative of those systems are'nickel-chromium-iron alloys of the type described in the above example or a high chromium (24-26 percent) nickel-containing (5-7 percent) stainless steels. There are systems in which the higher forging temperatures bring on the lowest flow stresses. A good balance can nonetheless be struck and a fine grained product of excellent strength, for example, can be derived, although the forging temperature is on the high side.

Stainless steels, nickel-base and copper-base alloysand alloys in which aluminum, titanium, zinc, magnesium and zirconium form the base thereof are contemplated within the scope hereof. The compositions given herein are intended to be by way of example only. A suitable powder stainless steel contains about 15 percent to 35 percent chromium; up to about 12 percent, e.g., 2 percent to 10 percent nickel; up to 1 percent or 1.5 percent titanium; up to 1 percent vanadium; up to 0.5 percent and preferably from 0.025 percent to 0.15 percent or 0.25 percent, oxygen; up to about 0.25 percent carbon, up to 1 percent silicon, up to 1 percent manganese, up to 3 percent or 4 percent molybdenum, up to 2 percent cobalt, up to 2.5 percent or 3 percent copper. Such steels can be forged at l,700 to 2,000 F but a lower forging temperature of 1,750 F. to l,850 F. is preferred, the alloy structure being comprised of a ferritic matrix throughout which austenite is dispersed. A particularly satisfactory stainless steel contains from 25 percent to 31 percent chromium and from 5.5 percent to 7 percent nickel. Other high chromium (10 percent or more) iron-base alloys can be employed.

A suitable nickel-chromium alloy contains from about 25 percent to 50 percent, e.g., 32 percent to 40 percent, chromium; up to 25 percent, e.g., about 14 percent to 22 percent,iron; up to about 1 percent, e.g.,

about 0.2 percent to 0.8 percent, titanium; carbon up to about 0.1 percent; up to 10 percent cobalt; up to about 1 percent, e.g., 0.2 percent to 0.8 percent, silicon, and from 20 percent to 60 percent nickel. Other elements, columbium, vanadium, aluminum, etc., can

be present. The alloy used in the above described example responds to this compositional range and the preferred forging temperature is from l,900 to 2,100 F.

Multiplex copper-base alloys include those containing from about 8 percent to 14 percent aluminum with or without up to 6 percent, e.g., 3 percent to 5 percent iron. Other illustrative copper alloys are those formed using substantial amounts of zinc, e. g., from 35 percent to 50 percent zinc, or magnesium in percentages from 8 percent to 12 percent. A particularly suitable nickelzinc-copper alloy contains from 4 percent to 71 percent, preferably 8 percent to 40 percent nickel, 29 percent to 40 percent zinc, balance copper and small amounts of other elements. These latter alloys can be forged from 900 to 1,050" F. where they have an alpha (face-centered-cubic) matrix with a dispersed beta phase (body-centered-cubic or body-centeredtetragonal). The straight 38 percent to 50 percent 'zinc, balance essentially copper also have a face-centeredcubic alpha and body-centered-cubic beta (the higher temperature phase) structure. A suitable forging range is from 850 to l,200 F.

Appropriate aluminum-base powder alloys can contain about 30 percent to 35 percent copper (forgeable at about 430 to 540 C.) or 8 percent to 15 percent silicon with up to 6 percent copper (can be forged from about 475 to 525 C.). The aluminum-copper structure is composed of an equiaxed aggregate of an aluminum solid solution and CuAl With regard to magnesium powder alloys they may be comprised of from percent to percent nickel,(forgeable at 425 to 475 C.) or from 27 percent to 33 percent copper (also forgeable at 425 475 C.) or from percent to 35 percent aluminum. Alloys having from 4 percent to 8 percent zinc, zirconium up to 1 percent, e.g., 0.4 percent to 0.8 percent, balance essentially magnesium can be employed.

A titanium alloy consisting of from 4.percent to 8 percent aluminum, 2 percent to 6 percent vanadium, balance essentially titanium and impurities, an alloy forgeable within the range of 875 to 1,000 C. and having an alpha plus beta structure, is also within the invention. Illustrative zinc alloys include those with alu minum in varying percentages such as 0.3 percent to 0.7 percent; 3 percent to 8 percent; 20 percent to 25 percent; and from 37 percent to 42 percent. These four groups of alloys can be forged at about 15 to 25 C., 200 to 300 C., 210 to 260 C. and about 240 to 260 C., respectively.

The invention is deemed useful for producing a variety of forged products, including pump gears, pipe and valve fittings, diffusion vanes, grooved pulleys, various surgical instruments, musical instrument keys, etc.

As indicated above, virtually all systems will be of two phases at the temperatures of forging. But it should be mentioned that inclusions, e.g., sulphide and silicate inclusions, are not deemed to be metallic phases for purposes herein but can be present in small quantities which'do not have an adverse effect. Upon cooling from forging, the microstructures may contain one or more transformation products of at least one of the metallic phases.

Although the present invention has been described in conjunction with preferred embodiments, modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. For example, in addition to forging, the alloy powders can be extruded or otherwise worked. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In the process of powder metallurgy hot forging in which a preform of solid material is hot forged in a die, the improvement of facilitating the forging process in terms of reduced forging pressure and/or temperature as a result of achieving lower flow stress conditions during forging while providing for a forged product having good mechanical properties which comprises, hot forging an alloy powder preform of an alloy base selected from the group consisting of nickel, copper, aluminum, titanium, zinc, magnesium, zirconium and iron containing above about 10 percent chromium at a temperature at which the alloy is characterized by a microstructure consisting essentially of at least two metallic phases with each of the phases being present in a volume percentage of at least about 10 percent with the average grain size of each of the phases being at least about ASTM 10, the volume of the phases and grain sizes thereof mutually coacting to retard grain growth during recrystallization.

2. A process in accordance with claim 1 in which two metallic phases are present, one being face-centered cubic and the other being body-centered-cubic and each is present in a volume percentage of at least 20 percent and the preform is formed from prealloyed powder.

3. A process in accordance with claim 1 in which one of the metallic phases is present in a range of about to percent and the grain sizes of the metallic phases average at least about ASTM 12.

4. A process in accordance with claim 1 in which the alloy is a stainless steel.

5. A process in accordance with claim 4 in which the stainless steel contains from 15 to 35 percent chromium, about 2 to 12 percent nickel, about 0.025 to 0.25 percent oxygen, up to 0.25 percent carbon, up to 1.5 percent titanium, up to 1 percent vanadium, up to 1 percent each of silicon and manganese, up to 4 percent molybdenum, up to 2 percent cobalt, up to 3 percent copper, the balance being essentially iron.

6. A process in accordance with claim 11 in which the alloy is a nickel-chromium alloy having from 25 to 50 percent chromium, 14 to 22 percent iron, up to 1 percent titanium, about 0.025 to 0.25 percent oxygen, carbon up to about 0.1 percent, up to 1 percent silicon, up to 1 percent manganese, and the balance essential nickel.

7. A process in accordance with claim 1 in which the metallic phases are comprised of alpha and beta and in which the alloy base is copper.

8. A process in accordance with claim 7 in which the copper-base alloy contains up to 6 percent iron and one metal from the group consisting of zinc and aluminum, the zinc being from 35 to 50 percent and the aluminum being from 8 to 14 percent.

9. A process in accordance with claim 1 in which the alloy powders used in forming the preform are of a particle size not greater than about 275 microns with their outer surfaces being substantially enveloped by an oxide film not greater than about 10 microns in thickness, the particles being of irregular shape. 

2. A process in accordance with claim 1 in which two metallic phases are present, one being face-centered-cubic and the other being body-centered-cubic and each is present in a volume percentage of at least 20 percent and the preform is formed from prealloyed powder.
 3. A process in accordance with claim 1 in which one of the metallic phases is present in a range of about 55 to 75 percent and the grain sizes of the metallic phases average at least about ASTM
 12. 4. A process in accordance with claim 1 in which the alloy is a stainless steel.
 5. A process in accordance with claim 4 in which the stainless steel contains from 15 to 35 percent chromium, about 2 to 12 percent nickel, about 0.025 to 0.25 percent oxygen, up to 0.25 percent carbon, up to 1.5 percent titanium, up to 1 percent vanadium, up to 1 percent each of silicon and manganese, up to 4 percent molybdenum, up to 2 percent cobalt, up to 3 percent copper, the balance being essentially iron.
 6. A process in accordance with claim 1 in which the alloy is a nickel-chromium alloy having from 25 to 50 percent chromium, 14 to 22 percent iron, up to 1 percent titanium, about 0.025 to 0.25 percent oxygen, carbon up to about 0.1 percent, up to 1 percent silicon, up to 1 percent manganese, and the balance essential nickel.
 7. A process in accordance with claim 1 in which the metallic phases are comprised of alpha and beta and in which the alloy base is copper.
 8. A process in accordance with claim 7 in which the copper-base alloy contains up to 6 percent iron and one metal from the group consisting of zinc and aluminum, the zinc being from 35 to 50 percent and the aluminum being from 8 to 14 percent.
 9. A process in accordance with claim 1 in which the alloy powders used in forming the preform are of a particle size not greater than about 275 microns with their outer surfaces being substantially enveloped by an oxide film not greater than about 10 microns in thickness, the particles being of irregular shape. 